Method for applying corrosion-protective coating to joint between corrosion-protectively coated steel pipes

ABSTRACT

In a joint between welded end portions of corrosion-protectively coated steel pipes, a heat-shrinkable tube is positioned so as to cover the outer circumference of a welded portion and the outer circumferences of its adjacent portions, and the heat-shrinkable tube is thermally shrunk in a state in which a gap between the heat-shrinkable tube and the aforementioned portions is kept in a vacuum so that the aforementioned portions are corrosion-protectively coated with the heat-shrinkable outer layer of the heat-shrinkable tube thus thermally shrunk.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for applying acorrosion-protective coating to a joint between corrosion-protectivelycoated steel pipes which are used in pipelines for town's gas, citywater, petroleum, district heating steam, etc.

2. Description of the Prior Art

Generally, steel pipes are used for a piping arrangement for carryingtown's gas, city water, petroleum, steam, etc. Because a steel materialfor such a type steel pipe has a defect of being corroded easily underthe presence of water or oxygen, the surface of the steel pipe isgenerally corrosion-protectively coated with asphalt, coal tar enamel orpolyethylene (PE), epoxy, urethane, etc. to thereby provide acorrosion-protective coating layer (referred to as "factory coatinglayer" because the corrosion-protective coating layer is formed in afactory) on the surface of the steel pipe.

When such corrosion-protectively coated steel pipes havingcorrosion-protective coating layers respectively are used, there arisesa problem that the corrosion-protective coating layers are deterioratedby a high temperature upon welding when piping construction is performedby welding the respective end portions of the corrosion-protectivelycoated steel pipes. In order to prevent the deterioration of thecorrosion-protective coating layers, therefore, the corrosion-protectivecoating layers in the vicinity of the end portions of the twocorrosion-protectively coated steel pipes are removed in advance so thatafter the end portions are connected to each other by on-site welding, acorrosion-protective heat-shrinkable tube or a corrosion-protectiveheat-shrinkable sheet is set to cover the outer circumference of thewelded portion of the joint and the outer circumference of adjacentportions adjacent to the joint where the surfaces of the steel pipes areexposed because the corrosion-protective coating layers are removed.Then, the heat-shrinkable tube or sheet is heated by propane gas, or thelike, so as to be thermally shrunk to thereby corrosion-protectivelycover the welded and adjacent portions with the heat-shrinkablematerial. The corrosion-protective coating method for on-site pipingconstruction is very excellent in corrosion protectivity and a lot ofpractical examples have been provided because the corrosion-protectivelycoated steel pipes can be protected from corrosive factors such aswater, air, etc. and can be prevented from being corroded by taking thelap between the corrosion-protective coating layer and theheat-shrinkable material sufficiently.

In the aforementioned corrosion-protective coating method, however, thesurfaces of the portions which are necessary for corrosion protection,that is, the flat portions (the surface of the corrosion-protectivecoating layer and the steel pipe surfaces after the removal of thecorrosion-protective coating layer) in the two end portions of thecorrosion-protectively coated steel pipes, the thickened portion in thewelded connection portion, the level-difference portions (between thesteel pipe surface and the corrosion-protective coating layer) generatedby the removal of the corrosion-protective coating layer, and otherportions, are not always smooth and are not even. Accordingly, air oftenremains in the portion thickened by welding, the level-differenceportions, etc. when the corrosion-protective heat-shrinkable tube, orthe like, is thermally shrunk. Furthermore, also in the flat portions ofthe steel pipe surface, the corrosion-protective coating layer, etc. airvoid is not avoidable at the time of thermally shrinking. Although theamount of such remaining air is too small to cause a problempractically, there are a feeling of uneasiness about progress ofcorrosion caused by the remaining air, a feeling of uneasiness aboutsafety of strength of the air-remaining portion against mechanical loadsuch as impact, or the like, and so on.

Various methods have been therefore carried out to eliminate airremaining, or the like, in the corrosion-protective coating portion ofthe heat-shrinkable material. For example, a method in which thelevel-difference portion is filled in advance with a material such as asealing material, or the like, mainly containing butyl rubber or amethod in which a hot-melt material mainly containing asphalt or butylrubber is applied in advance onto the flat portions of the steel pipesurface, the corrosion-protective coating layer, etc., is employed. Ineach of the methods, however, air void cannot be perfectly preventedfrom remaining. Furthermore, in the latter method, there arise a problemof gas generation and a problem of safety in dealing withhigh-temperature fluid, or the like, because melting and application ofthe hot-melt material are necessary in on-site piping construction.

SUMMARY OF THE INVENTION

The present invention is based on such circumstances and an objectthereof is to provide a method for applying a corrosion-protectivecoating to a joint between corrosion-protectively coated steel pipeswith a heat-shrinkable material without any air remaining in acorrosion-protective coating portion of the heat-shrinkable material.

According to the present invention, there is provided a method forapplying a corrosion-protective coating to a joint between welded endportions of corrosion-protectively coated steel pipes, comprising thesteps of: removing corrosion-protective coatings of the steel pipes atend portions thereof so as to expose surfaces of the steel pipes;welding the end portions of the steel pipes; positioning aheat-shrinkable tube so as to cover an outer circumference of a weldedportion of the joint and outer circumferences of adjacent portionsadjacent to the joint in which the corrosion-protective coatings areremoved; evacuating a gap between the heat-shrinkable tube and each ofthe welded and adjacent portions into a vacuum state; and heating theheat-shrinkable tube to thermally shrink thereof.

That is, the present inventors have made a series of researches toprevent air from remaining in the corrosion-protective coating portionwith the heat-shrinkable material when the joint iscorrosion-protectively coated with the heat-shrinkable material afterthe respective end portions of the corrosion-protectively coated steelpipes are connected to each other by on-site welding in a state in whichthe surfaces of the steel pipes are exposed by removing thecorrosion-protective coating layers in the vicinity of the respectiveend portions of the corrosion-protectively coated steel pipes inadvance. As a result, it has been found that air does not remain in thecorrosion-protective coating portion when the heat-shrinkable materialis thermally shrunk in a vacuum state. The founding has reached thepresent invention. Particularly, when an adhesive agent layer is formedon the inner circumferential surface of the heat-shrinkable tube, theheat-shrinkable tube is heated to be thermally shrunk in a vacuum stateand, at the same time, the heat-shrinkable tube is stuck to the surfaceof the welded portion of the joint between the steel pipes and to thesurfaces of the pipes in portions adjacent to the joint, by the actionof the adhesive agent layer so that the remaining of air can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is an explanatory view showing an actual example of aheat-shrinkable tube used in the present invention;

FIG. 2 is a partly sectional view of the heat-shrinkable tube;

FIG. 3 is an explanatory view showing a vacuum chamber used in thepresent invention;

FIG. 4 is a perspective view showing an upper split mold of the vacuumchamber;

FIG. 5 is an explanatory view showing a procedure of the presentinvention;

FIG. 6 is an explanatory view showing a procedure of the presentinvention;

FIG. 7 is an explanatory view showing a procedure of the presentinvention; and

FIG. 8 is an explanatory view showing a procedure of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described below in detail.

The method according to the present invention uses a heat-shrinkabletube positioned on the outer circumference of a welded portion of ajoint and on the outer circumference of adjacent portions adjacent tothe joint where corrosion-protective coatings are removed to expose thesurfaces of steel pipes, a device for heating the heat-shrinkable tubeto thermally shrink it, and a vacuum keeping device for keeping a gapbetween the heat-shrinkable tube and each of the welded and adjacentportions in a vacuum state.

As the aforementioned heat-shrinkable tube, any tube may be used so longas the tube has heat shrinkability. Preferably, a plastic tube havingheat shrinkability is used. Examples of such a plastic material includepolyethylene, modified polyethylene, polyvinyl chloride, polypropylene,ethylene-vinyl acetate copolymer, fluororesin such aspolytetraflouroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ethercopolymer, etc. Further, as the heat-shrinkable tube, it is possible touse a material shaped like a tube initially, a material shaped like atube by sewing opposite end portions of a belt-like matter together tojoin the end portions, A material shaped like a tube at the time ofcarrying out this method by chucks provided in opposite end portions ofa belt-like matter, and so on.

Further, when an adhesive agent layer is formed on the innercircumferential surface of such a heat-shrinkable tube, there arises anadvantage that not only the residue of air in the aforementioned weldedand adjacent portions can be reduced but also the heat-shrinkable tubeis stuck to the aforementioned welded and adjacent portions tofacilitate the formation of a coating. This adhesive agent layer may beformed by a method in which the adhesive agent layer is formedsimultaneously with the heat-shrinkable tube upon formation of theheat-shrinkable tube, a method in which an adhesive agent is applied tothe inner circumferential surface of the heat-shrinkable tube after theformation of the heat-shrinkable tube, or the like. Further, as theadhesive agent, it is possible to use any one of a pressure-sensitiveadhesive agent, a hot-melt adhesive agent, a heat-hardening adhesiveagent, a mastic type adhesive agent, and so on.

Further, in the case where a current-conduction type heater is disposedin the inside of the heat-shrinkable tube or on the outercircumferential portion thereof, the heat-shrinkable tube can be made togenerate heat internally easily and is easy in handling. As such acurrent-conduction type heater, not only a linear material such ascopper wire, or the like, but also a stratified matter having heatingwire such as copper wire, or the like, embedded therein are used. Thatis, in the case where the current-conduction type heater is disposed inthe inside of the heat-shrinkable tube, a method of making heating wiresuch as copper wire, or the like, contained in the heat-shrinkable tube,or the like, is carried out. In the case where the current-conductiontype heater is disposed on the outer circumferential portion of theheat-shrinkable tube, a method of laminating the stratified matterhaving heating wire such as copper wire, or the like, embedded in theoutside of the heat-shrinkable tube, or the like, is carried out. In thecase where heating wire is disposed in the inside of the heat-shrinkabletube, the heating wire is preferably disposed so as to be bent zigzagalong the circumference of the heat-shrinkable tube in order to followthe heat shrinkage of the heat-shrinkable tube. Contrariwise, in thecase where the current-conduction type heater is disposed on the outercircumferential portion of the heat-shrinkable tube, any suitable heatercan be used no matter it can follow the heat shrinkage of theheat-shrinkable tube or not. Further, a constituent material of theaforementioned stratified matter is not limited specifically. Forexample, plastics or rubber can be used.

As the device for heating the aforementioned heat-shrinkable tube tothermally shrink it, a heating device such as an electrothermic heater,or the like, produced to be separate from the heat-shrinkable tube maybe used so as to be disposed in the vicinity of the heat-shrinkabletube. Further, as described above, the current-conduction type heatermay be made to be contained in the heat-shrinkable tube, and so on. Inthis case, the heat-shrinkable tube is provided as an internal heatingtype tube, so that the number of parts is reduced and the work islightened compared with the case where a heating device separate fromthe heat-shrinkable tube is used.

As the device for keeping the gap between the heat-shrinkable tube andeach of the aforementioned welded and adjacent portions in a vacuumstate, a close device of an integral-type may be used or a closed deviceof a split-type in which the device is divided into two or three parts,or the like, may be used. As the device for evacuating such a vacuumkeeping device, it is possible to use a vacuum pump, or the like.Further, evacuation by the evacuating device needs to be continued untilthe heat shrinkage of the heat-shrinkable tube is completed. Althoughthe degree of vacuum is not limited specifically because it can beselected suitably correspondingly to various conditions, it is generallyselected to be not more than about 200 Torr, preferably in a range offrom 1 to 100 Torr. Further, it is preferable to use two evacuatingdevices. With use of two evacuating devices, the inside of theaforementioned split-type device is evacuated to a predetermined degreeof vacuum by the first evacuating device and the current-conduction typeheater is made to generate heat in the state of a vacuum, and if thedegree of vacuum is lowered because of leakage, or the like, from theseal portion of the vacuum keeping device in the duration of temperaturerising of the current-conduction type heater due to its heat generation,the degree of vacuum in the closed device can be recovered to and keptin the initial value by operating the second evacuating device.

The mode of carrying out the present invention will be described belowon the basis of the drawings.

FIGS. 1 and 2 show an actual embodiment of the heat-shrinkable tube usedin the present invention. This heat-shrinkable tube 1 is composed of anouter layer 2 (constituted by polyethylene, or the like, as describedabove) having heat shrinkability, copper wire 3 contained in the outerlayer 2, an inner layer 4 constituted by an adhesive agent layerprovided on the inner circumferential surface of the outer layer 2, anda separator 5 temporarily attached to the inner circumferential surfaceof the inner layer 4 so as to be able to be separated freely. In thedrawings, the reference numeral 6 designates lead wires; and 7,connection terminals.

FIG. 3 shows a vacuum chamber (vacuum keeping device) 10 used in thepresent invention. The vacuum chamber 10 shaped like a cylinder of thetype divided into two parts is constituted by a pair of upper and lowermolds 11 and 12 each shaped like a semicylinder. As shown in FIG. 4, aflange portion 13 is formed in the lower end surface of the upper molds11 so as to project. O-ring mount grooves 15 are formed in the lowersurfaces (which abut on the outer circumferential surfaces ofcorrosion-protective coating layers 24 and 25 of corrosion-protectivelycoated steel pipes 22 and 23 which will be described later) of twoflange portions 13a (one of which is formed in one end portion but ishidden so as not to be seen) formed in end portions perpendicular to thelongitudinal direction of the flange portion 13. On the other hand, aflange portion 14 facing the flange portion 13 of the upper mold 11 isformed in the lower end surface of the lower mold 12 so as to project inthe similar manner to the upper mold 11. O-ring mount grooves are formedin the lower surfaces of flange portions 14a formed in end portionsperpendicular to the longitudinal direction of the flange portion 14(see FIG. 7). Further, fixing means are formed in two places on each oftwo side portions 14b along the longitudinal direction of the flangeportion 14 of the lower mold 12. Each of the fixing means comprises ablock 17 fixed to the lower surface of the flange portion 14b, aU-shaped fixture 18 having a lower piece 18a of the U-shape attached tothe block 17 so as to hinge freely, and a clamping screw rod 19 screwedinto a screwhole (not shown) formed in an upper piece 18b of the U-shapeof the fixture 18. The flange portions 13 and 14 of the upper and lowermolds 11 and 12 are stuck to each other by screwing the clamping screwrod 19 so that the vacuum state of the inside can be kept.

Using such a aforementioned heat-shrinkable tube 1, the vacuum chamber10, two vacuum pumps (not shown), etc., the joint betweencorrosion-protectively coated steel pipes 22 and 23 can becorrosion-protectively coated as follows. That is, as shown in FIG. 5,corrosion-protective coating layers 24 and 25 at the respective endportions of the two corrosion-protectively coated steel pipes 22 and 23to be joined to each other are first removed, and then the two endportions of the steel pipes are joined to each other by on-site welding.In the drawing, the reference numeral 26 designates a welded portion;and 27 and 28, adjacent portions where surfaces 22a and 23a of the steelpipes are exposed. Then, as shown in FIG. 6, the heat-shrinkable tube 1from which the separator 5 is separated is positioned and set so as tocover the welded portion 26, the adjacent portions 27 and 28 andportions of the corrosion-protective coating layers 24 and 25 near theadjacent portions 27 and 28. Four connection terminals 7 (of which twoconnection terminals are shown in the drawing) of the heat-shrinkabletube 1 are connected to output terminals 20a (of which two outputterminals are shown in the drawing) of an electric source controller 20.Then, as shown in FIG. 7, the vacuum chamber 10 is set (in thisoccasion, O-rings 21 are mounted into the O-ring mount grooves 15 of theupper and lower molds 11 and 12). Then, a first one of the two vacuumpumps is connected to the vacuum chamber 10 and operated (made into acommunicated state) to evacuate the vacuum chamber 10 into a vacuumstate (for example, 60 Torr or less). In this state, the electric sourcecontroller 20 is turned on to make an electric current flow in a copperwire 3 so that the copper wire 3 generates heat. When the degree ofvacuum in the vacuum chamber 10 is lowered (for example, to 100 Torr) bythe rising of the temperature in the inside of the vacuum chamber 10because of the heat generation, the second vacuum pump is connected tothe vacuum chamber 10 and operated (made into a communicated state) torecover and keep the degree of vacuum to the value obtained at the timeof operating the first vacuum pump. Further, the outer layer 2 isthermally shrunk by the heat generation due to current-conduction sothat the aforementioned portions 26 to 28, and so on, arecorrosion-protectively coated. After the current conduction and theoperation of the vacuum pumps are then stopped, the vacuum chamber 10 isremoved and, the lead wires 6 of the heat-shrinkable tube 1 are cut off.In such a manner, as shown in FIG. 8, the joint between thecorrosion-protectively coated steel pipes 22 and 23 can be coated with aheat-shrinkable material.

A resultant product of the present invention will be described below. Inthis product, town's gas pipes each having a diameter of 750 mm andhaving a corrosion-protective coating layer of polyethylene on its outercircumferential surface are used as the corrosion-protectively coatedsteel pipes 22 and 23. The corrosion-protective coating layers at therespective end portions of the two pipes are removed in advance by 150mm in the longitudinal direction of the pipes. Further, aheat-shrinkable tube with an inner diameter of 850 mm and with a lengthof 600 mm comprises a heat-shrinkable outer layer of polyethylene(thickness: 1.5 mm, heat shrinking rate: 50%), a pressure-sensitiveadhesive agent layer (thickness: 1.5 mm) provided on the innercircumferential surface of the outer layer, and a separator temporarilyattached to the adhesive agent layer. Further, a copper wire iscontained zigzag in the outer layer. Lead wires are connected to thecopper wire and, further, connection terminals are provided to the leadwires (the structure of the heat-shrinkable tube is as shown in FIGS. 1and 2).

After the joint of the town's gas pipes is corrosion-protectively coatedwith the outer layer 2 by the above-mentioned method, there is no airremaining in the welded portion, the steel pipe surfaces, thelevel-difference portions, etc. of the joint. That is, there is obtaineda coating state incomparably better than the conventional technique. Theeffect of the corrosion-protective coating method, according to thepresent invention, by using the heat-shrinkable tube in a vacuum systemis apparent.

As described above, in the method for applying a corrosion-protectivecoating to a joint between corrosion-protectively coated steel pipesaccording to the present invention, air is prevented from remaining inthe corrosion-protective coating portion of the heat-shrinkable materialeven in the case where the joint is corrosion-protectively coated withthe heat-shrinkable material after the end portions of thecorrosion-protectively coated steel pipes are connected to each other byon-site welding in a state in which the steel pipe surfaces are exposedby removing in advance the corrosion-protective coating layers in thevicinity of the end portions of the corrosion-protectively coated steelpipes. In the present invention, particularly, when an adhesive agentlayer is formed on the inner circumferential surface of theheat-shrinkable tube, the heat-shrinkable tube is heated to be thermallyshrunk in a vacuum state and, at the same time, the heat-shrinkable tubeis stuck to the surface of the welded portion of the joint between thesteel pipes and to the surfaces of the pipes in portions adjacent to thejoint, by the action of the adhesive agent layer so that the remainingof air can be reduced. Further, when two vacuum generators are connectedto the split molds to keep the gap between the heat-shrinkable tube andeach of the welded portion of the joint and its adjacent portions in avacuum after the split molds are disposed on the outer circumference ofthe heat-shrinkable tube, one of the two vacuum generators is operatedto evacuate the inside of the split molds to a vacuum when the inside ofthe split molds is to be evacuated and the heat-shrinkable tube isthermally shrunk in that state, and lowering of the degree of vacuum inthe split molds at the time of thermal shrinking is recovered by theoperation of the other vacuum generator to thereby keep the degree ofvacuum in the initial value. Thus, the degree of vacuum can be recoveredto the initial value even in the case where the degree of vacuum in thesplit molds is lowered at the time of thermal shrinking of theheat-shrinkable tube.

What is claimed is:
 1. A method for applying a corrosion-protectivecoating to a joint between welded end portions of corrosion-protectivelycoated steel pipes, comprising the steps of:removingcorrosion-protective coatings of said steel pipes at end portionsthereof so as to expose surfaces of said steel pipes; welding said endportions of said steel pipes together; positioning a heat-shrinkabletube so as to cover (1) an outer circumference of a welded portion ofsaid joint, and (2) outer circumferences of pipe portions adjacent tosaid joint and from which said corrosion-protective coatings areremoved; disposing split molds, which are capable of keeping a gapbetween said heat-shrinkable tube and each of said welded and saidadjacent pipe portions in a vacuum, on an outer circumference of saidwelded portion and on outer circumferences of said adjacent pipeportions so that said welded and said adjacent pipe portions arepositioned in an inside of said split molds; connecting two vacuumgenerators to said split molds; operating a first vacuum generator toevacuate said inside of said split molds; heating said heat-shrinkabletube to thermally shrink said heat-shrinkable tube onto said joint andsaid adjacent pipe portions; and operating a second vacuum generator torecover lowering of a degree of vacuum in said split molds at the timeof thermally shrinking said heat-shrinkable tube to thereby keep saiddegree of vacuum at its initial value.
 2. The method according to claim1, wherein an adhesive agent layer is provided on an innercircumferential surface of said heat-shrinkable tube.
 3. The methodaccording to claim 1, wherein a linear current-conduction type heaterfor heating said heat-shrinkable tube is disposed on an outercircumference of said heat-shrinkable tube to meander along acircumference of said heat-shrinkable tube, and the method furthercomprises the step of removing said linear current-conduction typeheater after said heat-shrinkable tube is thermally shrunk.
 4. Themethod according to claim 1, wherein a linear current-conduction typeheater for heating said heat-shrinkable tube is buried in a wall of saidheat-shrinkable tube to meander along a circumference of saidheat-shrinkable tube.
 5. The method according to claim 1, wherein amaterial of said heat-shrinkable tube is selected from polyethylene,modified polyethylene, polyvinyl chloride, polypropylene, ethylene-vinylacetate copolymer and fluororesin.
 6. The method according to claim 5,wherein said fluororesin is selected from the group consisting ofpolytetrafluoroethylene and tetrafluoroethylene-perfluoroalkyl vinylether copolymer.
 7. A method for applying a corrosion-protective coatingto a joint between welded end portions of corrosion-protectively coatedsteel pipes, comprising the steps of:removing corrosion-protectivecoatings of said steel pipes at end portions thereof so as to exposesurfaces of said steel pipes; welding said end portions of said steelpipes together; positioning a heat-shrinkable tube so as to cover (1) anouter circumference of a welded portion of said joint, and (2) outercircumferences of pipe portions adjacent to said joint and from whichsaid corrosion-protective coatings are removed; disposing split moldswhich are capable of keeping a gap between said heat-shrinkable tube andeach of said welded and said adjacent pipe portions in a vacuum, on anouter circumference of said welded portion and on outer circumferencesof said adjacent pipe portions so that said welded and said adjacentpipe portions are positioned in an inside of said split molds;connecting two vacuum generators to said split molds; operating a firstvacuum generator to evacuate said inside of said split molds, such thatsaid gap between said heat-shrinkable tube and each of said welded andsaid adjacent pipe portions is in a vacuum state; heating saidheat-shrinkable tube with a heater to thermally shrink saidheat-shrinkable tube onto said joint and said adjacent pipe portions;operating a second vacuum generator to recover lowering of a degree ofvacuum in said split molds at the time of thermally shrinking of saidheat-shrinkable tube to thereby keep said degree of vacuum at an initialvalue; and removing said heater after said heat-shrinkable tube isthermally shrunk.